1. Field of the Invention
The present invention relates to a control for an internal combustion engine provided with a cylinder halting mechanism.
2. Description of the Related Art
There has been an internal combustion engine provided with a plurality of cylinders and a cylinder halting mechanism for halting a part of the plurality of cylinders according to an operating condition of the engine. In such an internal combustion engine, when the number of operating cylinders is decreased, an air-fuel ratio of an exhaust atmosphere may become thin (lean) if air is supplied while fuel injection is stopped in a cylinder to be newly halted. Such lean atmosphere may lower the emission.
Japanese Patent Application Laid-Open (JP-A) No. 2000-170560 discloses a technique where, in order to prevent the lean exhaust atmosphere, an intake valve of a cylinder to be halted is stopped when a predetermined time elapses after an exhaust valve of the cylinder is stopped.
In the above technique, the exhaust valve is stopped prior to the stop of the intake valve to prevent the air from discharging, so that an exhaust air-fuel ratio can be prevented from becoming lean. However, the intake/exhaust valve stop timing possibly fluctuates according to a response characteristic of a mechanism which controls operations of the intake and exhaust valves. Therefore, a deviation may occur between fuel injection stop timing and the intake/exhaust valve stop timing. Particularly, in the case where the intake and exhaust valves are hydraulically operated, there is a probability that such fluctuations may easily occur in the response characteristic. If a deviation occurs between the fuel injection stop timing and the intake/exhaust valve stop timing, the exhaust air-fuel ratio may become lean, which may lower the emission. Accordingly, there is a demand for a new technique of effectively suppressing the lean exhaust air-fuel ratio caused by such a deviation.
Furthermore, in the above technique, in order to more surely prevent the lean exhaust air-fuel ratio, a fuel is increased to an operating cylinder to enrich an air-fuel ratio of the operating cylinder. In this regard, there is a room for improving the fuel consumption.